Digital Holographic Microscopy, DHM, is a known imaging technique allowing field depth constraints of conventional optical microscopy to be overcome. Schematically, it consists of recording a hologram formed by interference between the light waves diffracted by the observed object and a reference wave having spatial coherence. A general introduction to digital holographic microscopy can be found in the article by Myung K. Kim titled Principles and techniques of digital holographic microscopy published in SPIE Reviews Vol. 1, N° 1, January 2010.
It has recently been proposed to use digital holographic microscopy for automated identification of microorganisms. For example, the article by N. Wu et al. titled Three-dimensional identification of microorganisms using a digital holographic microscope published in Computational and Mathematical Methods in Medicine, Vol. 2013, art. No. ID 162105, describes a method to identify different types of bacteria in a volume to be analysed by means of numerical propagation towards the plane corresponding to a focus on the particle. The images focused at different depths are used to reconstitute a three-dimensional representation of the microorganisms. These are then classified using non-linear 3D filtering.
Similarly, the article by Ahmed El Mallahi titled Automated three-dimensional detection and classification of living organisms using digital holography microscopy with partial spatial coherent source: application to monitoring of drinking water resources published in Applied Optics, Vol. 52 No. 1, January 2013, describes a method comprising a first step to detect the position of the bacteria in the volume to be analysed, a focusing step at different depths in the volume using numerical propagation, followed by classification of the bacteria from their morphological characteristics.
The aforementioned identification methods are complex, however, insofar as they require focusing in successive focus planes. A contrario, focusing in a single focus plane, in other words at a single analysis depth, is generally insufficient for the identification of a type of microorganism with low false detection rate. It is therefore the objective of the invention to propose a method for identifying organic particles by digital holographic microscopy which allows a low false detection rate to be obtained whilst being simple and robust.